Minnesota traded for Wiggins. Cleveland drafted Wiggins then traded him for Love and did win a Championship a couple of years later.
And who's saying it happens every time. The point is from a mathematical standpoint.
The NBA has the smallest rosters so from that perspective one or two great draft picks makes the biggest impact to your roster.
Also, your starters are on the court for what, 35-40 of the 48 minutes in a game? They're out there for basically 80% of the game...unlike hockey where your best players are on the ice for maybe 33%...and baseball, where a position player usually gets 4 AB's a game and it's completely random as to how much they get to impact a given game defensively. In basketball, your starters are involved more than in other sports. So yes, from a mathematical standpoint, the right pick or two can impact your roster very quickly. Now, it's up to your scouting department to make sure that happens of course...the but if they do their job right, they can make a bigger impact, faster, than in other sports.
As to whether it works every time? Of course it doesn't. Nothing in this world is 100%. Thinking that is retarded. But I stand by the theory that a good draft or two can impact an NBA team much faster than in other sports.